1. Field of the Invention
The present invention relates to phase change material microcapsules synthesized by using a PMMA prepolymer and an organic-solvent free process, and more particularly to phase change material microcapsules synthesized by an organic-solvent free process which enables a polymer monomer is subject to pre-polyerization to form Polymethyl methacrylate prepolymer and also to encapsulation and polymerization at an oil-water interface, thereby achieving the preparation of the phase change material microcapsules.
2. Description of Related Art
In general, phase change material has the ability to change its physical state at a certain temperature, and absorbs or releases a large amount of latent heat in phase changing process. One of the characteristics of phase change material is during the change of physical state, the temperature of the material itself remained almost unchanged in the completion of the phase changing process. The combination of the above features favors to a broader development of the phase change material in various applications such as food preservation and temperature regulation.
The phase change material has a good advantage in energy storage. However, if it is directly applied in the phase transition, material loss during the phase transition will greatly affect the life of the material.
A microcapsule is a core-shell technology which uses a trace material as a shell to encapsulate another materiaml Therefore, the use of microencapsulation technology can solve the above mentioned problems of material loss and volume change caused by physical changes. In addition, the particle size becomes very small and the specific surface area becomes larger after the microencapsulation providing a larger heat transfer area. After the microencapsulation, the capsulate wall is usually thin which renders the heat transfer greatly improved.
Preparation of Polymethyl methacrylate (PMMA) microcapsules has been proposed in a lot of relevant literatures. For example, Park et al disclosed in 2006 modified titanium dioxide (TiO2) is encapsulated with Polymethyl methacrylate by using electrophoretic suspension process to form multi-core titanium dioxide microspheres. In 2007, Zhang et aml disclosed that with a phase separation method, Polymethyl methacrylate and PS are used to encapsulate tetradecane, and the obtained microcapsulates have uniform size and average particle size of 1-2 μm. In 2010, Ma et al, disclosed that the use of UV light to irradiate over a light initiator initiates the polymerization of methyl methacrylate (MMA) within an emulsion, obtaining phase-changing microcapsulates of good thermal stability. In 2011, Alkan et aml successfully encapsulate eicosane of particle size between 0.4-2.3 μm with Polymethyl methacrylate by using emulsion polymerization process, obtaining microcapsules of good thermal stability and chemical stability. In Taiwan, Zhang disclosed that methyl methacrylate and an initiator are subject to pre-polymerization in an organic solvent; the phase change material and the previously obtained prepolymer are added to an aqueous solution of polyvinyl alcohol (PVA); and then emulsification, homogeneous mixing, warming polymerization, filtering, washing, and drying are carried out to obtain microcapsules. Some approaches of preparation of phase change material microcapsules have been proposed in the industry, such as textiles Industry Research Institute disclosed a process to prepare phase-changing microencapsulated by using an acrylic monomer of bi-functional or tri-functional group.
Except Zhang's process that encapsulates the phase change material with the prepolymer at two steps, most of disclosures describe that methyl methacrylate monomer is directly subject to polymerization for encapsulating the phase change materiaml The approaches proposed by the Textile Research Institute and Zhang prepare the prepolymers in the environment containing organic solvents. The organic solvents in the manufacturing process are, eventually, directly discharged into the atmosphere, which has an adverse influence on the working environment.
For this reason, the inventors have made intensively efforts for years over the aforementioned problem and finally succeed to develop phase change material microcapsules synthesized by using a PMMA prepolymer and an organic-solvent free process, which overcomes the prior shortages.